1. Field of the Invention
The present invention relates generally to a magneto-optical recording medium, and more particularly to a magneto-optical recording medium improved in recording magnetic field sensitivity and reproducing characteristics.
2. Description of the Related Art
A magneto-optical disk has received attention as a memory medium that becomes a core in the recent rapid development of multimedia, and it is desired to have a larger recording capacity. The recording capacity of a magneto-optical disk may be increased by a method of reducing the width of a groove on a substrate to thereby increase a radial recording capacity or by a method of reducing the length of a recording mark to thereby increase a tangential recording capacity.
A method of recording marks on a magneto-optical disk is generally classified into a light intensity modulation recording method and a magnetic modulation recording method as known in the art. The light intensity modulation recording method is performed by first ordering the direction of magnetization to a downward direction, for example, before recording (i.e., erasing data) and next directing a pulsed laser beam according to information to be recorded in the condition where an external magnetic field is applied in an upward direction. By applying the upward external magnetic field to a region where the temperature has been raised by irradiation with the laser beam, the magnetization in this region only can be oriented in the upward direction, thereby forming a recording mark according to the information to be recorded in this region.
On the other hand, the magnetic modulation recording method is performed by first continuously directing a laser beam to raise the temperature of the disk up to a temperature at which the coercive force of a recording layer is lowered, and next applying an external magnetic field larger than or equal to the coercive force as modulated upward and downward. By modulating the external magnetic field according to information to be recorded, the magnetization in the corresponding region can be oriented in the upward or downward direction, thereby forming a recording mark according to the information to be recorded.
It is known that the magnetic modulation recording method is advantageous over the light intensity modulation recording method as the method of recording a shorter mark. To reduce the recording mark length, high-frequency recording is required. However, increasing a recording frequency in the magnetic modulation recording method is accompanied by a reduction in effective current flowing in a magnetic head due to high-frequency loss, causing a problem that a sufficient magnetic field cannot be generated. Further, the distribution of a magnetic field generated from the core of the recording head is not uniform, so that there arises another problem that an effective recording magnetic field required for recording is reduced. Also in the light intensity modulation recording method, there arises a problem that when a recording magnetic field and an erasing magnetic field are large, the power consumption by a magnetic field generating circuit is increased to cause an increase in size of the magnetic field generating circuit. Accordingly, it is desirable to reduce the recording magnetic field and the erasing magnetic field.
A technique for improving recording magnetic field sensitivity is described in Japanese Patent Laid-open No. 5-298764. In this technique, a magnetic film having a composition such that rare earth sublattice magnetization is dominant at room temperature is used as a recording layer, and a rare earth-transition metal amorphous alloy film having a composition such that transition metal sublattice magnetization is dominant at a temperature for forming a recording magnetic domain in the recording layer is formed on either side of the recording layer. However, this method cannot improve a C/N characteristic.
Further, a method of reducing the diameter of a beam spot is known as the method for increasing the recording capacity. The beam spot diameter can be reduced by shortening the wavelength of a laser beam used for recording and reproduction from a conventional wavelength of 780 nm or 680 nm generally used in the related art or by enlarging the numerical aperture (NA) of a lens from a conventional NA of 0.55. However, in the case of using this method, there occur a noise increase due to an increase in power density in the beam spot and a carrier reduction due to a reduction in reproducing power (a reduction in light quantity) in the beam spot, thus causing a degradation in C/N characteristic.